doc: remove confusion about count and for_each

writing-terraform-configurations.md

@@ -40,3 +40,22 @@ website = {
 }
 ```
 {% endcode %}
+
+## Use `for_each` and `count` for different purposes
+
+If you want to create multiple resources in a loop for each element in a list or map, always use `for_each`. It will save you if you want to remove some resources dependent on an element in the middle of a list. 
+
+```hcl
+resource "aws_instance" "this" {
+  for_each = toset(var.availability_zones)
+  availability_zone = each.value
+  // ... other attributes
+}
+```
+If you want to have resource only if a variable seted to true, use `count`
+```hcl
+resource "aws_instance" "vpn" {
+  count = var.use_vpn ? 1 : 0
+  // ... other attributes
+}
+```